CLEMENCY (15, 112 mins) Drama/Romance. Alfre Woodard, Wendell Pierce, Aldis Hodge, Richard Schiff, Richard Gunn, Danielle Brooks, Alex Castillo. Director: Chinonye Chukwu.

Released: July 17 (UK & Ireland, selected cinemas)

Life and death are confirmed by last-minute telephone calls in Clemency, a quietly devastating drama told through the eyes of a death row inmate bound for the chamber and a long-serving warden, who must remain emotionally detached until the final injection of potassium chloride stops heart function.

Writer-director Chinonye Chukwu wanders the same echoing corridors as Dead Man Walking, The Green Mile and Just Mercy, exploring different facets of the American criminal justice system.

Her second feature is emboldened by a fearless central performance from Alfre Woodard as the sleep-deprived warden, who is as much a prisoner of her hulking facility as hundreds of men in her care.

Over the course of two riveting hours, Woodard chips away at her character's armour, which she wears to protect against visible twinges of doubt, until trickles of saltwater break through and smear her unmovable, cold facade.

Her omission from this year's Oscar nominations was an injustice.

Aldis Hodge also delicately reaches into our chests to rip out our hearts with a measured supporting performance as a prisoner, who has always pleaded his innocence.

In one horrifying sequence, he repeatedly pounds his forehead against a wall, determined to seize control of his destiny once hope of a stay of execution has been extinguished.

"I say when I die!" he screams forlornly as guards race into the blood-smeared cell to restrain him.

Chukwu opens to sobering and gut-wrenching effect with the bungled execution of Victor Jimenez (Alex Castillo).

The first injection of midazolam is supposed to render Jimenez unconscious but the prisoner fits violently as his parents and invited guests watch in wide-eyed horror through glass from an adjacent room.

Warden Bernadine Williams (Woodard) is haunted by Jimenez's final moments, writhing in agony on a gurney, and she repeatedly seeks solace in a local bar rather than in the arms of her husband Jonathan (Wendell Pierce).

"I don't see how it's going to work living with an empty shell of a wife," he pleads. "I need a pulse, Bernadine. I need to know you're still here."

His wife coolly prepares for the execution of prisoner Anthony Woods (Hodge), who has served 15 years for the murder of a police officer but has always asserted that his accomplice pulled the trigger.

Lawyer Marty Lumetta (Richard Schiff) hopes the governor might weigh up the evidence and grant Woods clemency.

"I am going to fight for him right up to the moment you stick that needle in his arm," Marty snarls at Bernadine, "Just so you know."

Clemency is a powerful character study, which delivers its knockout blows in prolonged silences on both sides of the sliding bars.

A telephone call between Woods and his estranged ex-girlfriend (Danielle Brooks) pulses with unvarnished raw emotion but it's Woodard who inevitably sears into the memory.

A three-minute close-up of her face finally registering everything we have been feeling, her posture perceptibly sagging as the piercing scream of a flatline on a heart monitor steadily increases in volume, is almost unbearable.